A microwave radio link or a radio link system is a communications system that uses a beam of radio waves to transmit signals or data between two locations. The locations may be of various distances apart. A simplified two-way radio link system 100 is illustrated in FIG. 1. The radio link system 100 comprises two radio link terminals/nodes 101 between which there is a data transfer system, for example a radio link/channel 102. The radio link 102 may comprise one or more carriers. Using the radio link node 101 shown at the left side of FIG. 1 as a reference point, the left radio link node 101 may be referred to as near end radio link node 101n, and the right radio link node 101 may be referred to as the far end radio link node 101f. The radio link system 100 comprises four main elements: two transmitters 103, two receivers 105, transmission lines 107, a radio link/channel 102 and antennas 110. The antennas 110 may be mounted on for example a base station tower. The transmitter 103 generates a microwave signal and modulates it with an input signal so that it conveys meaningful information to be communicated. Each radio link node 101 comprises a respective transmitter 103, a near end transmitter 103n and a far end transmitter 103f. The transmission line 107 carries the signal from the transmitter 103 to the antenna 110 and, at the receiving end of the link, from the antenna 110 to the receiver 105. The antennas 110 emit the microwave signal from the transmission line 107 onto the radio link 102. At the receiver site, an antenna 110 pointed toward the transmitting station collects the signal energy and feeds it into the transmission line 107 or processing by the receiver 105. The receiver 105 extracts the microwave signal and demodulates it into its original form. Each radio link node 101 comprises a respective receiver 105, a near end receiver 105n and a far end receiver 105f. 
In some embodiments, the transmitter 103 and the receiver 105 may be incorporated into one unit, such as a transceiver. In yet other embodiments, the transmitter 103 and the receiver 105 may also be incorporated with the antenna 110 and transmission line 107 into one unit.
A radio link system 100 may operates on the duplex principle, which means that the system 100 comprises to connected radio link terminals 101 that may communicate with one another in both directions, they use two separate frequencies for transmitting and receiving data. Thus, in a duplex system 100, radio link terminal 1, e.g. the near end radio link terminal 101n, would send information to radio link terminal 2, e.g. the far end radio link terminal 101f, on frequency F1 while radio link terminal 2 would send information to radio link terminal 1 on frequency F2.
Radio link systems 100 used in telecom transmission systems have traditionally been used as wire replacement for voice circuit connections, thus operating at a fixed rate and generally with a very high requirement on availability at the rate the radio link system 100 is dimensioned for. Up until recently, voice traffic has dominated the transmission networks but over the last few years there has been an enormous growth of packet data traffic in communication networks. Microwave radio links systems 100 have taken on the challenge to meet the increasing demands for data traffic by several means.
At first, methods to map data traffic on circuit connections were developed, known as Ethernet over Time Division Multiplexing (EoTDM). This provides interfaces for data traffic, e.g. Ethernet interfaces. The radio link systems 100 still work at fixed rates and with quite poor utilization of the radio channel. This is mainly due to the overhead necessary for bonding and hierarchical multiplexing of individual tributaries. The granularity provided by hierarchical multiplexing is yet a problem.
The next step was the development of optimized transport schemes where circuit connection may coexist with native packet data transport. These systems are known as hybrid systems. The main characteristics of these systems are that multiplexing is no longer hierarchical and that bandwidth may be allocated arbitrary for packet data. This takes away many of the problems with granularity and bonding overhead. The possibility to allocate bandwidth arbitrary also makes it easier to utilize the radio spectrum provided. Arbitrary bandwidth allocation also provides the possibility to reallocate bandwidth from circuit connections to packet data as demands change. Together with Ethernet switching functions and Quality of Service (QoS) prioritization functions, it also enables the possibility of Adaptive Modulation and Adaptive Coding (ACM) in order to improve spectrum utilization. ACM is a method for automatically adapting a bit-rate to current channel conditions by altering the code rate and/or modulation scheme. The basic concept of these methods is that the radio link system 100 is dimensioned for high availability at a committed rate consisting of circuit connections and/or a portion of the packet data traffic. When external conditions permit, e.g. defined by the receiver bit error rate or Signal-to-Noise Ratio (SNR) in the receiver 105, throughput may be increased by reducing the overhead for error correction coding and/or using a higher order modulation scheme.
With Long Term Evolution (LTE) emerging as the new mobile system generation, the need for circuit connected transmission will decrease. The most recent radio link systems 100 are designed for pure packet transport. Support for circuit connection is still provided in many cases for these radio link systems 100 but now as circuit emulated connections in the Ethernet packet network. With the transition to packet, the networks also get denser and the throughput and functionality increases.
In order to support higher modulation schemes and higher output power, the transmitters 103 and power amplifiers used in radio link systems 100 have also been developed to better efficiency.
Adaptive Transmitter Power Control (ATPC) is a method for controlling transmitters' output power using the far end receiver as detector. ATPC t is used for transmit power management both for pure packet, hybrid and pure circuit connection radio link systems 100. Recent development including predistortion for radio amplifiers has not only lead to reduced output power, but also to lower power consumption due to ATPC since the radios may operate in to class AB with still excellent linearity at system level. Radio amplifiers may be classified using the classes A, B, AB, C, D and E. The classes are based upon the conduction angle or angle of flow of the input signal through the (or each) output amplifying device, that is, the portion of the input signal cycle during which the amplifying device conducts.
Adaptive modulation makes sure that throughput for the available radio link 102 is maximized at every time. Still, in typical mobile packet networks the amount of data actually transported normally varies by a factor of ten between night and day. There is also a big variation in traffic between weekdays and weekends.
This means that if conditions are good at night when only a small amount of packet data traffic is transported in the network, the radio link system 100 will adapt to a high rate. I.e. most of the time only idle traffic will be transported.
In a modern radio link system 100, the power dissipated is partly proportional to the rate and partly proportional to the output power. For a constant SNR in the receiver 105, the required output power has in its turn an exponential relation to the rate. In addition to this there is also a static power dissipation. Equation 1 below shows an expression for this. P0 is the static power dissipation, ap and ae some proportional constants and B the exponential base for how power dissipation in the power amplifier (PA) scales with the rate.PD=P0+ap·r+ae·Br  (Equation 1)
In a typical radio link system 100 the proportional part scales by 10% and the exponential by 50% of the maximum power dissipation between the highest and lowest rates. If the rate r is expressed in bits/symbol, the B parameter may assume values between 1 and 2. B=1 corresponds to an amplifier having its power consumption constant with output power. B=2 corresponds to a hypothetical amplifier with constant efficiency with regard to output power. Real life Radio Frequency (RF) amplifiers using linearization techniques operate somewhere between class A and B, a.k.a. class AB and are located somewhere in between those extremes. But even with B=1.1, i.e. reducing the Power Amplifier (PA) output power by half will reduce the power consumption by 10%, there will be a significant scaling with rate in the PA power consumption.
In multiple carrier systems this is even more conspicuous; some of the carriers may at times transport idle traffic only. Regard e.g. a dual carrier system with adaptive modulation from 2-10 bits/symbol. If this system is running at its highest rate when the actual throughput is 10%, corresponding to one carrier running at its lowest rate, it consumes up to 85% of its power transporting idle patterns.
Power consumption is an important contributor to the cost of ownership for the transmission systems. When the networks grow denser, power consumption will become even more important. And it is not only the OPerating EXpenditures (OPEX) for electrical energy but also CApital EXpenditures (CAPEX) related to dimensioning of the power distribution and generation, e.g. cabling, solar cells, batteries, generators etc., that is affected. OPEX is the ongoing cost for running a system, and CAPEX is the cost of developing or providing non-consumable parts for the system.
From the example above, where 85% of the power was used to run idle traffic and with the knowledge that traffic during night, i.e. ⅓ of the time, is 10% of the traffic at day, the conclusion is that up to 30% of the power consumed in current packet radio link systems 100 is misused.
The above discussion focuses on reducing the amplifier output power in order to reduce power consumption, requiring a particular design and characteristics of the amplifier.